Conventional DC/DC converters are commonly used in various fields, and a boost DC/DC converter is known as one type of DC/DC converter.
FIG. 9 shows the configuration of a main part of a general boost DC/DC converter. The boost DC/DC converter (hereinafter referred to as “boost converter”) 100 of FIG. 9 boosts an input voltage Vin that is supplied from a DC power source 201 and supplies a boosted voltage to a load 202.
The boost DC/DC converter 100 is provided with an input capacitor C1, an inductor L1, a transistor T1, a diode D1, and an output capacitor C2. Operating as a switching device, the transistor T1 controls a current flowing through the inductor L1 according to a PWM signal that is supplied from a control circuit (not shown). The inductor L1 transmits power from the DC power source 201 to the output side according to an on/off operation of the transistor T1. The diode D1 is provided for rectification. The control circuit (not shown) controls the transistor T1 so that the output voltage is kept at a prescribed value. As a result, a voltage required by the load 202 is generated. The boost converter having the above configuration is described in JP-A-9-240422 (on FIG. 1 and paragraphs [0022]-[0024]).
Among DC/DC converters is one having a current limiting function of controlling the output current or the load current so that it does not exceed a prescribed threshold value. In the boost converter 100 shown in FIG. 9, the current limiting function can be realized by monitoring the output current and controlling the transistor T1 so that the output current does not exceed a prescribed threshold value. In this case, as shown in FIG. 10, when the output current has increased to a threshold value Ir, the operation mode is switched from a voltage control mode to a current limiting mode. In the current limiting mode, the current is limited by decreasing the output voltage.
However, in the boost converter 100 shown in FIG. 9, the input and the output are not insulated from each other but always connected to each other via the diode D1. Therefore, the output voltage cannot be decreased so as to be lower than the input voltage Vin. That is, as shown in FIG. 10, it is impossible to perform current limitation when the output voltage needs to be made lower than the input voltage Vin to limit the output current to a prescribed value.
The above problem can be avoided in insulated DC/DC converters using a transformer or the like. However, insulated DC/DC converters require a rectifier device having a high breakdown voltage and hence may suffer low conversion efficiency.